The last decades brought an exponential increase in mobile traffic volume. This will continue and a 1000-fold increase by 2020 has been forecasted. Small-cells are capable of enabling new services, increasing energy-efficiency, and reducing the costs of handling explosive data growth.
Due to strong inter-cell interference, small-cell deployments will require a higher degree of coordination than currently deployed systems. Small-cells may be deployed where it is difficult or too expensive to deploy fixed broadband access, optical fiber or line-of-sight (LOS) based microwave solutions for backhaul. The Broadband Forum (see http://www.broadband-forum.org for reference) reported that 30% of a mobile operator's OPEX today is spent for backhaul networks. Recently, wireless backhaul has received more attention due to its higher deployment flexibility and lower costs. The report “Wireless Backhaul: The Network Behind LTE, WiMAX, and 3G”, In Stat, October 2010 shows that the expenditures for wireless backhaul will increase by 41% from 2009 to 2014. Hence, small-cell deployments must be connected by heterogeneous backhaul technologies that consist of fiber, microwave solutions, as well as other forms of wireless backhaul (for reference see NGMN Alliance (Next Generation Mobile Networks), “Next Generation Mobile Networks Optimised Backhaul Requirements,” NGMN Alliance, August 2008).
So far, most radio access designs (including 3GPP architecture) consider the backhaul network to be sufficiently dimensioned (over-provisioned). While this is already challenging in today's backhaul networks, it might be even less true as we move towards small cells and more coordinated operation. Therefore, the backhaul must be considered as a limited resource when operating the radio access network. However, the 3GPP LTE mobile network architecture provides no means to take into account the underlying physical transport network and functional split of the physical implementation.
If a backhaul link is congested, this may be solvable in many cases by means of re-routing traffic over alternative paths if such exist. However, the closer to the base station, the less path diversity of the backhaul topology is typically available. Given two or more small cell base stations (or a combination of small cell and macro base stations) with overlapping coverage areas, the only remedy to relieve congested backhaul links might therefore be to enforce mobility of individual user terminals from one cell to another so that traffic destined for those terminals can be routed via a different and less congested part of the backhaul network.
In R. Ferrus, J. Olmos, and H. Galeana: “Evaluation of a cell selection framework for radio access networks considering backhaul resource limitations,” IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, Athens (Greece), September 2007, the authors discuss a cell selection framework for radio access networks which considers backhaul resource limitations. By means of an analytical framework, they show the benefits of taking into account backhaul limitations for the selection of radio access nodes. In fact, the document discloses the use of a very general formulation through a multi-dimensional Markov chain to show that treating all backhaul-resources as one pool of resources (trunk pool) will increase the overall capacity (trunking gain) that may be assigned to different cells.
WO 2009/067297 A1 discloses a cellular communication system that performs serving cell management in response to the backhaul loading of the base stations of the system. To this end, the base stations are equipped with a buffer including a plurality of sub-buffers for buffering backhaul data and with means for determining backhaul loading congestion. The system considers the backhaul load from a base station perspective, which renders the system rather inefficient and inflexible.